JPS5912632Y2 - Cable sheath surge shielding coefficient measurement circuit - Google Patents

Description

[Detailed description of the invention] The present invention relates to a cable sheath surge shielding coefficient measurement circuit, and is useful for measuring the shielding coefficient of a cable having an electromagnetic shielding sheath with a good conductor inner layer and a ferromagnetic outer layer structure. be.

BACKGROUND OF THE INVENTION As a method of reducing induced disturbances caused by power lines and AC/DC electric railways to communication cables, an electromagnetic shielding sheath having a good conductive inner layer and a ferromagnetic outer layer is generally used around the outer periphery of the cable.

The shielding coefficient η of this electromagnetic shielding sheath is defined by the cable length l, the angular frequency ω, the DC resistance of the good conductor inner layer R8·l, and the external impedance of the cable sheath/earth return circuit (jωL
o+Ro)・l, the sum of the grounding resistances at both ends of the cable sheath is Re−l, and the additional impedance due to the ferromagnetic outer layer is (
J rj7fX)・l, it is given by.

Therefore, the shielding coefficient is generally determined by the additional impedance (ar+j, ix
) is obtained by measuring and substituting the value into equation (1).

However, even if the additional impedance is measured using continuous waves in this way, it is not possible to calculate the shielding coefficient against surges using the same method as described above.

Firstly, surges have a continuous spectrum from direct current to high frequency as a frequency component, and secondly, surges have a continuous spectrum from direct current to high frequency.
This is because a ferromagnetic material is used for the cable sheath, so there is non-linearity in the characteristics, and thirdly, the external impedance (jωLo+Ro) of the cable sheath/earth return circuit also has frequency characteristics.

For this reason, an appropriate method for measuring the shielding coefficient against surges has not yet been announced, and, of course, a measuring circuit for the same has not yet been announced.

Therefore, it is impossible to accurately measure the shielding coefficient against surges.

Therefore, an object of the present invention is to provide a surge shielding coefficient measuring circuit that can easily and accurately measure the surge shielding characteristics of a cable sheath under any installation conditions.

The structure of the present invention that achieves this purpose is to provide an equivalent external impedance of an appropriate value to the cable in order to accurately measure the shielding coefficient of the cable sheath when changing the surge application waveform, cable sheath structure, grounding resistance, etc. The cable is inserted in series with the cable sheath, and a surge is applied to it by a low output impedance surge power supply, thereby causing a current similar to that in the actual installation state to flow through the cable sheath.

Embodiments of the present invention will be described in detail below based on the drawings.

As shown in Figures 1a and b, a cable sheath 1, which is a sample with an effective length l', has an equivalent resistance R, an equivalent inductance L, and a copper cylinder for the return circuit connected in series to simulate a cable sheath/earth return circuit. 2 gives the form of a closed circuit.

At this time, the equivalent resistance R and the equivalent inductance L are made equivalent to the external impedance and the ground resistance.

The surge power supply 3 is a low-impedance power supply that applies a surge voltage corresponding to the induced voltage when not shielded as a constant voltage source.

Further, the cable sheath current flowing through the closed circuit is measured by a current measuring probe 4.

The cable sheath inner surface electric field measuring terminal 5 connects the cable sheath 1 with a copper wire 6 for measuring the cable sheath inner surface electric field extending inside the cable sheath 1 whose one end is connected to the equivalent inductance L and extending toward the other end. The cable sheath 1 is connected to the other end, so that the voltage at both ends of the cable sheath 1 can be measured.

The applied voltage measurement terminal 7 has one end connected to the power supply side of the equivalent resistance R, and connects the cable sheath 1 and the return copper cylinder 2.
The space between is connected to the applied voltage measurement electric wire 8 extending toward the other end and the other end of the return copper cylinder 2, and therefore the equivalent resistance R, equivalent inductance L, and The voltage across the cable sheath 1 can be measured.

Here, the procedure for determining the equivalent resistance R and equivalent inductance L will be explained.

The external impedance of the actual cape sheath/earth return circuit (jωLo+R).

) has been studied in detail so far, and the resistance R.

and inductance L.

is known to vary greatly depending on frequency.

Therefore, this cannot be incorporated into the shielding coefficient measuring circuit of this embodiment.

Therefore, in this embodiment, the resistance Ro and inductance L are constant with respect to the frequency so that the cable sheath current equivalent to the cable sheath current flowing in the actually installed cable flows.

is selected and the external impedance (Ro+jωLo) is incorporated into the shielding coefficient measuring circuit of this embodiment as an equivalent external impedance.

Therefore, the equivalent resistance R in this shielding coefficient measuring circuit is equal to the equivalent external resistance Rol' per measured cable sheath length and the ground resistance R per measured cable sheath length.

l', and the equivalent inductance L is obtained by subtracting the inductance between the cable sheath 1 and the return copper cylinder 2 of the measurement circuit from the equivalent external inductance Lol' per measurement cable sheath length.

As an example, the procedure for determining the equivalent resistance R and equivalent inductance L in the case of a cable strung at a height of 5 m above ground on the ground with a ground conductivity of 10 m/'v/m will be explained.

A ground return circuit in which an aluminum cylindrical cable sheath with an outer diameter of 2ri = lQmm and a thickness of 0.2mm is grounded with a grounding resistance R8 = 2m, Q/m, a peak voltage lv/m per unit cable length, a waveform of 10X40μs, 40X 160μ
The cable sheath current was measured when a voltage of S, 60 x 240 μs was applied.

In this case, the current shown by the solid line in FIG. 2 flows through the cable that actually spans the ground.

On the other hand, the external resistance R. =1mJ7/m, external inductance L.

= 2.1 μH/m, the current shown by the dotted line in Figure 2 flows, and the same current flows as in the actual earth return circuit, so this external resistance R.

and the external inductance L.

The values of can be taken as the equivalent resistance and equivalent inductance of the external impedance.

Further, when the inner diameter 2r2 of the return copper cylinder 2 of the shielding coefficient measuring circuit of this embodiment is 9Q mm, the inductance L' per unit length between the cable sheath 1 and the return copper cylinder 2 is as follows.

In this way, the values of the equivalent intance L and equivalent resistance R of the shielding coefficient measuring circuit are determined.

In addition, when a concentric return copper cylinder 2 as shown in FIG. 1 is used as a return path for the cable sheath current, the inductance changes depending on the outer diameter of the cable sheath 1 between the cable sheath 1 and the return copper cylinder 2. and external inductance L.

Since the changes in are the same, their effects are canceled out, and even if the outer diameter of the cable sheath 1 changes, it can be measured with the same equivalent resistance R and equivalent inductance L.

When the ground conductivity and the wiring method are different, the equivalent external impedance for each case may be obtained in the same manner as described above, and the equivalent resistance R and equivalent inductance L may be determined.

This makes it possible to measure the cutoff coefficient for all environmental conditions.

In order to measure the shielding coefficient with such a shielding coefficient measuring circuit, by applying a surge voltage corresponding to the induced voltage in the unshielded state using the surge power supply 3, a cable sheath current equivalent to that in the case of actual installation is caused to flow (the cable sheath current is The inner surface electric field of the sheath 1) x (length l' of the cable sheath 1), the applied voltage, and the cable sheath current are measured.

That is, the electric field measurement terminal 5 on the inner surface of the cable sheath allows ■
A8(t) = (inner surface electric field of cable sheath 1) (V
/m) X (Length l' of cable sheath 1) (m
), the applied voltage measuring terminal 7 measures Vco(t)=applied voltage (■), and the current measuring probe 4 measures I(t)=cable sheath current (A).

At this time, if the measurement cable length is l' (m), then V1 (
t)=VCD(t)/l'(V/m) when not shielded (7)
Voltage between core wire and earth per unit cable length, V2 (
t) -VAB(t)/l'(V/m) is the core wire per unit cable length of the installed shielded cable.
This is the cable-sheath voltage.

Also, the voltage between the core wire and the ground zero potential per unit cable length of the installed shielded cable is V3 = (Ro-I(t) + vAB(t)/l') (v/m)... ...(4) Te is expressed.

Therefore, the shielding coefficient of the voltage between the core wire and cable sheath is expressed as the ratio of the voltage peak value: peak value of V2(t)/Vl
The peak value of V3(t) and the shielding coefficient of the voltage between the core wire and the ground zero potential are also expressed as the ratio of the voltage peak value: peak value of V3(t)/V. It is expressed by the peak value of (t).

As described above in detail with the embodiments, according to the present invention, the surge shielding characteristics of a cable in any installation state can be easily and accurately measured indoors using a short cable sheath.

Therefore, it is possible to efficiently examine installation conditions such as effective shielding structures and grounding methods.

Furthermore, since the induced voltage waveform in the shielded cable core can be directly measured, it is extremely useful when examining the shielding characteristics of the sheath against the induction of discontinuous waveforms.

[Brief explanation of the drawing]

FIG. 1a is a circuit diagram showing a surge shielding coefficient measuring circuit according to an embodiment of the present invention, FIG. 1b is a sectional view taken along the line X-X, and FIG.
The figure is a graph comparing the cable sheath current waveform in an actual earth return circuit and the cable sheath current when external impedance is simulated using equivalent resistance and equivalent inductance. In the drawing, 1 is a cable sheath, 2 is a return copper cylinder, 3 is a surge power source, 4 is a probe for current measurement, 5 is a terminal for measuring an electric field on the inner surface of the cable sheath, and 7 is a terminal for measuring applied voltage.

Claims (1)

[Scope of utility model registration request] Measurement of a short cable sheath as a sample, a return copper cylinder concentrically surrounding the cable sheath, a surge power supply that generates a surge voltage equivalent to the induced voltage when unshielded, and the current flowing at this time. the external impedance of the cable sheath/ground return circuit, the grounding resistance, and the cable sheath/ground return circuit when a surge voltage occurs.
A closed circuit in which an equivalent resistance corresponding to the inductance between the return path copper cylinders and an equivalent inductance are connected in series, a cable sheath inner surface electric field measurement terminal for measuring the voltage across the cable sheath, and the equivalent resistance. , an applied voltage measurement terminal for measuring the equivalent inductance and the voltage across the cable sheath.